ENERGY

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ENERGY
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• TYPES OF ENERGY
• Kinetic - energy in motion
• Potential - stored energy
• Chemical - energy stored in the bonds of
  molecules and atoms
• Nuclear energy- energy stored in the nucleus of
  atom
• Electrical energy- electrons moving through a
  wire
• Mechanical, sound, motion, thermal and radiant
  are other examples

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Energy Efficiency
percentage of energy input that does useful work
in an energy conversion system
www.bio.miami.edu/beck/esc101/Chapter1415.ppt
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When were energy efficient we use less energy to
do something as good as before or better. We have
to remember there is a limited amount of
non-renewable fuel sources such as coal and gas.
Even if we dont run out of fuel, we can damage
our environment by using too much and wasting
energy.
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Energy transformations are great, but very
inefficient! Our bodies are about 20 efficient,
and burning the gasoline in your car is only 10
efficient! Most of the energy is lost in the form
of heat!
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Efficiency of Some Common Devices

• Device Efficiency ()
• Dry-cell flashlight battery 90
• Home gas furnace 85
• Storage battery 70
• Home oil furnace 65
• Small electric motor 62
• Steam power plant 38
• Diesel engine 38
• High-intensity lamp 32
• Automobile engine 25
• Fluorescent lamp 22
• Incandescent lamp 4

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UNITS OF ENERGY

• JOULE (J) or kilojoule (kJ)
• Calorie (cal) amount of heat required to raise
  the temp of 1 g of water 1C
• BTU (British thermal unit) amount of heat
  required to raise the temperature of 1 pound of
  water 1 F Used in US to rate water heaters,
  furnaces and air conditioners. (coal)
• Therm (thermal units) often used by gas companies
  (natural gas)

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CONVERSIONS

• 1 cal 4.186 joules
• 1 BTU 1.05 kJ 252 cal
• 1 therm 100,000 BTU
• 1 kWh 3413 BTU
• 1 cubic foot of gas contains 1031 BTUs
• 1 ton of coal contains 2.5 x 107 BTUs

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POWER- THE RATE AT WHICH ENERGY IS USED

• P E/t E energy t time
• THE UNIT FOR POWER IS THE WATT
• 1 W 1 J/s
• A 100 watt light uses 100 J/s of electrical
  energy
• Power (watts) current (amps) x voltage(volts)

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FIRST AND SECOND LAWS OF THERMODYNAMICS

• If a bulb is 20 efficient, then the bulb
  converts 20 of the electrical energy into light
  and 80 is lost as waste heat.
• First Law of Thermodynamics
• Energy can be converted from one form to another
  ( electrical ? heat)
• Second Law of Thermodynamics
• In any energy conversion, some energy is
  converted into lower quality energy

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Knowing the relationship between energy and power
allows you to find the energy used when an
appliance of known power (watts) operates for a
known amount of time.

• EXAMPLE how much energy (kJ) does a 75 watt
  light bulb use when it is turned on for 25
  minutes?
• ANSWER E Pxt E 75 J/s x 60s/min x 25 min
  110,000 J or 110 kJ

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The kWh (kilowatt hour)

• Kilowatt hour is a unit of energy. This is the
  term used by the electric company.
• E P x t
• 1 kWh is equal to 1 kW (1000 W) delivered
  continuously for 1 hour (3600 seconds)
• 1 kWh 1000 J/s x 3600s 3,600,000 J or 3.6 e6
  J or 3600 kJ

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• 1 kWh is the energy required to power 10 100-watt
  light bulbs for 1 hour.
• The average US home uses about 10,000 kWh
  electric energy/year.

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• Remember your metric conversions!
• GMKHD DCMMNP
• 5000 w ____ kW
• 4800 J ____ MJ
• 88 kc ____ cal

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Energy Calculations Worksheet

• Take a look and highlight terms.
• See if you can do the first problem tonight.

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Energy Calculation Answers

• 1. A.) 100 kg x 1kcal x 20 C 2,000kcal
• kg/C
• B.) 2,000 kcal x 4.186j x 1,000 cal 8.37
  MJ
• cal
  1kcal
•  
• C.) 1Btu x 8.37 x 106J 7934Btus
• 1,055J
•  
• D.) 7,934Btu x 1h x 60m 11.9
  min
• 40,000Btu hr

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• 2. A.) 1KWH x 3.6 x 10J x 1BTU
  3,412Btu
  KWH 1,055J
• B.) The second law of thermodynamics prevents
  100 conversion of heat to mechanical or
  electrical energy. A typical coal fired plant
  operates at 33 efficiency, meaning that only 1/3
  of the energy in coal is converted to
  electricity.

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• 3.A.
• 120MBtu x 1,000,000 1.2x10 Btu x 1 ft3 x
  1/ccf 1,165ccf (gas)
• 1 MBtu
  1030Btu 100ft3
• 
  
• B. 1165ccf 1,942ccf x 0.9 1,748 to heat
  house_at_ 60 eff
• .6 ccf 
• C. 1165 1456.25 ccf x 0.9 1,310.6 to heat
  house_at_ 80 eff
• .8 ccf
• Money saved per year in heating costs
  1748-1,310.6 437.40
• Years to payback cost of furnace 4000/437.40
  9.1 years

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• 4.A.)
• 8760hrs/yr x 160 W/m2 x 10m2 x .20 2,803.2 KWH
• 2,803.2 KWH x 3.6x106 J x 1BTU 9,565,421 BTU
  9.57 MBT
  KWH 1055 J
• B.) 2,803.2 KWH .2803 28
• 10,000 KWH
•  
• 9,565,421 Btu 1,200 BATHS
• 7,934 Btu/BATH

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• 5.A.) 250 W/m2 x 10m2 x .10 x 1KW x 8,760HR
  2,190 KWH
• 1,000W
• B.) 2,190 KWH .219 x 100 21.9
• 10,000KWH
•  
• C.) 10,000KWH x 10m2 45.6621m2
• 2,190KWH
  

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• 6. 8,760 x 200W/m x 106m x .001 x 3.4123 Btu
  498 Tons
• 12 X 106 Btu/ ton
  W

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• 7A.) 1,000MW x 1,000KW
• MW
  4,000 Wind Turbines
• 250 KW
• B.) Solar energy can help to decrease the total
  amount of energy provided by the power company by
  21.9 . Solar energy used to heat water could be
  feasible for small families. It would not be
  economical or practical to use solar energy as
  your only energy source at this time. Wood is
  very inefficient as a heat source. The forest in
  example 7 could provide enough energy for almost
  50 homes. The trees would require continuous
  replenishment the soil would require
  fertilization to maintain

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• 8.) Battery 4.5 WH .0045KWH
• 1,000W/KW
  
•  
• 1 222.22 / KWH
• 0.0045KWH

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• 9.
• Natural gas cost is 1.14 x 1cf x
  1 ccf 11.07/MBTU
• ccf 1030 BTU 100cf
•  
• 120/.8 x 11.07 1660.50
•  
• Propane gas cost
• 1.69 x 1 gal x 1x 106 BTU
  18.37/MBTU Gal 92000 BTU
  MBTU
•  
• 120/.8 x 18.37 2775.

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• Fuel Oil cost
• 1.93 x 1 Gal x 1x106 BTU
  14.51/MBTU
• Gal 133,000 BTU MBTU
•  
• 120/.8 x 14.51 2177
•  
• Electricity
• 0.10 x 1KWH x 1 x 106 BTU
  29.51/MBTU
• KWH 3412 BTU MBTU
•  
• 120/.8 x 29.51 3,517

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SAMPLE PROBLEMS!

• 1. Dr. Jones Nov-Dec power bill shows that his
  home used 1355 kwh over a 30 day period.
• How much energy was used (kJ) for the 30 day
  period?
• Find the energy used in
  J/day.
• At the rate of .0749/kwh,
  what is the doctors power
  bill (w/o tax)?

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• A. 1355 kWh x 3600kJ/1 kWh 4878000kJ ( 4.9 e 6
  kJ)
  
• B. 1355 kWh/30 days x 3600000 J/1kWh 1.6e10
  J/day
• C. 1355 kWh x 0.0749/kWh 101.49

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• 2. The current through a toaster (110 V) is 8
  amps
• A. What is the power (in watts) of the toaster?
• B. How much energy (J) will
• the toaster use in 5 minutes of
• operation?

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• A. W 110 V x 8 A 880 W 880 J/sec
• B. E P x t so,
• 880 J/sec x 60 sec/1 min x 5 min 264,000 J

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• 3. A 100 watt light bulb is 20 efficient.
• A. how much energy does it use in 12 hours of
  operation?
• B. How much energy does the
• bulb convert into light over the
• 12- hour period?
• C. How much energy does the
• bulb convert into heat over the
• 12-hour period?
• D. Convert the total energy use into kwh.

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• A. 20 J/sec x 60sec/1min x 60 min/1 hr x 12 hrs
  
• 864,000 J
• B. 864,000 J
• C. 80 J/sec x 60 sec /1min x 60min/1hr x 12 hrs
  3,456,000 J
• D. 4,320,000 x 1kwh/3.6e6J 1.2 kwh

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• 4. An electric clothes dryer has a power rating
  of 4000W. Assume that a family does 5 loads of
  laundry each week for 4 weeks. Also assume that
  each load takes 1 hour.
• Find the energy used in both J
• and kJ
• B. If the cost of electricity is
• 0.758/kwh, find the cost of
• operating the dryer for 3 months.

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A. E P x T 4000J/sec x 60 sec/1min x 60
min/1hr x 20 hrs x 1kw/3.6e6 B. 80 kwh x
0.0758/kwh 6.06
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• 5. Dr. Nicks natural gas bill states that his
  household used 110 therms of energy for a 30-day
  period.
• Convert 110 therms to kwh
• His charge for the energy
  
• was 88.78. Find the cost
• of this natural gas in /kwh.

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1. 110 therms x 100,000 BTUs/1 therm x 1.05kJ/1BTU x
   1 kw/3500kJ 3208.3 kwh
2. 88.78/3208.3 kwh 0.03 kwh

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